The present invention relates to a semiconductor device and to a method of manufacture thereof. Particularly, the invention relates to a semiconductor device of the type sealed with resin and an art that can be effectively adapted to a method of manufacture thereof.
A semiconductor device of the type sealed with resin is usually obtained by bonding a pellet onto a tab that serves as a pellet-mounting portion in a lead frame, by electrically connecting the pellet to inner ends of the leads by wire bonding, and by molding an epoxy resin or the like thereon.
In order to decrease thermal resistance of semiconductor devices and to increase their resistance to humidity, it is accepted practice to fabricate a base member of lead frame using copper, which has good thermal conductivity and a coefficient of thermal expansion which is close to that of the sealing resin, and to fabricate the lead frame using a copper alloy which is chiefly composed of copper.
With the lead frame made of copper alloy, however, the surface is easily oxidized by heating while assembling the device. The film of copper oxide is easily peeled off from the surface of base member composed of copper alloy. When the resin sealed-type semiconductor device employing such a lead frame is to be sealed by the molding of resin, therefore, the sealing resin tends to peel off from the surface of the lead frame due to poor adhesiveness between the copper alloy base member and the sealing resin. Therefore, moisture and contaminating substance infiltrate into the package through poorly adhered portions or peeled portions; i.e., the semiconductor device loses its resistance to humidity. In other words, the copper alloy forming the lead frame is easily oxidized by heat in the assembling step, and the film of copper oxide having poor adhesiveness easily peels off from the base member composed of copper. Therefore, a gap develops on the interface between the sealing resin and the lead frame which permits water and other contaminants to enter. Accordingly, the semiconductor device of such a structure has poor resistance to humidity and is therefore unreliable.
In order to improve adhesiveness between the lead frame and the sealing resin, therefore, it has been attempted to prevent the oxidation from taking place on the surface of the copper base member of the lead frame by plating a tin-nickel alloy or an iron-nickel alloy on the whole surface of the lead frame. These alloys exhibit sufficiently great adhesive strength relative to the resin even when their surfaces are oxidized in the heating step.
The above-mentioned example in which a tin-nickel alloy is plated onto the lead frame that employs copper as a base material, has been disclosed, for example, in Japanese Patent Laid-Open Nos. 81777/1979 and 129976/1979.